Since no one else is replying I'll use this thread for an associated topic.

Several postings on other threads have use terminology from and examples of camera lenses to illustrate thoughts on telescopic sight discussions. There are some important differences in both purpose and design of these instruments.

A modern camera consists of two principal components: an imager - film or digital sensor and the compound camera lens. The camera lens is a focal (having a defined focal length) instrument in that it is designed to project a real image of the subject on the full frame of the imager.

Telescopic sights (as well as telescopes, spotting scopes, binoculars and monoculars) are aids to the eye (magnifiers) to better see distant objects. These are all afocal (with an undefined focal length) instruments that are designed to present a magnified virtual image to the eye.

The eye is the original "camera" which has a compound lens (cornea and interocular) and an imager (retina).

Virtual images need lenses to create them by focusing near parallel light rays from each point of the target to converge back to a single point. If those light rays don't meet at one point, the image will be blurred and fuzzy at its intended plane.

The virtual image presented by an afocal instrument may be view by an eye or photographed by a camera but it cannot be projected such as onto a screen.

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Virtual images cannot be made unless light rays from something are focused (converged) to a single point. Lenses do that.

If a virtual image can be seen or photographed, it can be projected onto a screen. All three systems are identical in the way an image is focused on an eye retina, film or screen. Only difference is the focal lengths of the lenses and the distances between each three parts.

Optical engineers use formulas to calculate lens focal lengths and their positions in the scope for mechanical engineers to design the scopes lens mount positions and their mechanical adjustment ranges. Lens makers mold, grind then coat and sometimes cement them together to meet size and focal length requirements. Instrument machinists make the mechanical parts.

A rifle scope's front objective lens focuses (converges) diverging light rays from the target to a virtual image behind it at the 1st image plane. If it's a 7" focal length and target's at a 1000 yards, a virtual target image is 7.001361" behind it. Focused on a 100-yard target, that virtual image is 7.013638" behind it. Exact same optical principals of a camera with a 177.8028mm (7 inch) lens.

That scope's erector lens group's focal length is what's needed to converge the 1st inverted virtual image plane's diverging light rays to focus and converge them on the reticle at the 2nd image plane as a virtual, twice inverted target image. Exact same optical principals of a camera with a lens with a fraction of an inch focal length.

An eyepiece lens behind the reticle and 2nd image plane has a focal length equal to its distance back from that plane. It converges the diverging light rays from that 2nd image plane to near parallel paths best suited to the aiming eye properties. The aiming eye sees that now magnified image of the target as the eye lens focuses those light rays to converge its image inverted on the retina. Another optical system like cameras have. The human brain inverts it upright.

Scope magnification is equal to its objective lens' focal length divided by the optical focal length of the erector and eyepiece lenses combined. If the scope objective lens has a focal length of 7 inches and its power is 10X, the optical focal length of those combined lenses behind the 1st image plane is 7/10ths inch. If the scope's 16X, those combined lenses focal length is 7/16ths inch.

Human eyes' concave retina compensates well for its lens focus field curvature. Image sharpness is virtually equal over its 120 to 150 megapixel rod-cone sensors. Focused image distance is virtually the same for its sensors for all viewing distances.

Another Update on post #3 above - reply to an email question that may be helpful to all:

To clarify the normal use definition of magnification:

As used for a camera, magnification is the ratio of the real object size to the size of the object on the imager (digital sensor or film).

As used for telescopic sights, spotting scopes, telescopes, binoculars and monoculars, magnification is the ratio of size of the object in the virtual image as seen through the instrument to the size of the object as seen by the naked eye.

Good explanation of Afocal, Fred. I thought you meant the entire lens system.

An Afocal lens system is what most variable power scopes use for their erector lens pair. They separate to different spacing and their midpoint moves back and forth to change power and image size at the eyepiece/ocular lens focus plane.

The decades old standard for camera lens magnification has been used lens focal length divided by its "standard" lens' focal length. 35mm camera lens standard is 50mm as first set by Leica. If a 200mm lens is used, its images are 4 times bigger than the standard lens makes. 2-1/4" square Rolleiflex, 80mm. 4x5" sheet film old press camera, 135mmm (my 53 year old Graflex Super Speedgrafic).

Regarding camera lens magnifying images.....
A 2" (about 50mm) focal length camera lens puts images of something 100 yards away about 1/1800th (.000555) their size on its film/sensor. That subject is about 1800 times larger than its image in the camera.

The above is the compensated zoom system typical of high end zoom scopes. What it also illustrates is the paraxial rays at the input side after modification (changes to magnification) emerge as paraxial rays at the output which defines an Afocal System.

Another way of defining an Afocal System is one in which both the input and output are focused at infinity (paraxial in and out in the above illustration). Hence, the system has no defined focal length and is therefore an Afocal System.

Not really, at least not for this forum and not for most discussions that involve rifle scopes. I find that when it comes to a discussion of the topic of rifle scopes you can pretty much break down the participants into one of two categories...those that believe or at least will say that scopes "gather" light, and those that emphatically deny it is possible. Your attachment kind of implies that it does, but doesn't use the exact terminology. Which is something else I have always noticed about this topic...whenever scopes and optics in general are discussed there is this big play of words and their meanings. Gather, enhance, concentrate.......for some reason the use of these words sets people off in a bad way.
Personally, I am sitting on the fence about this. When you take a magnifying glass and start a fire something is being concentrated/enhanced/gathered. Then there is that other argument that you only have so much light, i.e. at dusk, to pass it thru several lenses and the amount left that got thru has to decrease......
All that said, I expect all of my scopes to show me what I cannot see with just my eye at dusk and they do....go figure!!!! I really don't care how it works or what the proper use of the english language is to accurately describe it as long as it works.

Since no one else is replying I'll use this thread for an associated topic......Several postings on other threads have use terminology from and examples of camera lenses to illustrate thoughts on telescopic sight discussions. There are some important differences in both purpose and design of these instruments.

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Exactly my point......

About the only thing I have found to be "cut in stone" regarding optics...optics are like hookers...the more money you spend the better they work.

I believe there is a typo or misspelling in the last sentence of the PDF (emphasis added):

" ... companion term lightness is used for image impression as a hole."

"whole"?

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Good catch, yes I did mean whole. This illustrates two possible problems:
1. I can still think faster than I can type error free.
2. My spell checker doesn't know the difference either (AI is no better than HI).

I have no formal qualifications on the visual system, only the auditory one.

The thread title caught my eye as I had an interesting experience the other day. It is mid winter here in New Zealand and I took advantage of a warmer day to test my new 6BR. It wears a 5-25x52 Z5 Swaro and this was the first time I've ever experienced a target image so bright, it was both unpleasant and close to washed-out in terms of aiming point definition. The scope was on max magnification and the sun angle wasn't even close to behind me. I had to basically squint to make the brightness tolerable. Didn't stop me shooting some very tight groups but I just know I wouldn't have experienced that bright an image in most other scopes, even in my ATAC-R NF.

I'm used to high end scopes but wasn't expecting that level of brightness from a 25mm scope tube on 25X. Normally brightness correlates reasonably well with definition but this level wasn't helpful. I'm sure rabbits on dusk will stand out nicely though

You are a perfect example of what I was trying to get high power scope users to understand in my initial post. Thank you for posting your experience. While it is summer here in Wisconsin, I've experienced the same issue shooting over snow in our winters.

Be advised that when exposed to such high luminance levels, your eye will initially react by closing the pupil (not good as that will cause increased diffraction fringing within the eye) then begin accommodation by reducing the sensitivity of the photochemical cone cells (also not good as this works differently for the three color sensing cones producing an effective change in your color perception). This reaction is reasonably quick with most of the change occurring in the first minute but full accommodation taking about ten minutes. However, recovery to normal eye response functionality will be 20 to 30 minutes dependent on your age.

On very bright days I start with a ND2 (0.5 transmittance) Neutral Density filter on my objective but I've used ND4 (0.25 transmittance), ND8 (0.125 transmittance) and even stacked a ND2 with ND8 for
ND16 (0.0625 transmittance). My goal is to reach an exit pupil luminance that won't close down my pupil too far and allow me to look through the scope without producing the glare reaction.

Added 6/26 --- Your scope has a 54x0.75 mm objective female thread and readily available ND filter closest size is 55x0.75 mm male so if you wish to try ND filters you will need to also get a 54 to 55 mm step-up ring.

ChrisNZ, pardon my profound ignorance of the land of the long white cloud, but looking at Wikipedia, it shows that temperatures in July/August seem to be in the low to mid-teens centigrade. I always thought you needed 0 or less centigrade to have snow on the ground, but maybe things are different in the southern hemisphere.

So reading your post, I am puzzled as to what could be causing such a bright target image, with the Sun overhead (well as much as it gets during the day in winter in the southern hemisphere.) The size of the main tube has nothing to do with the amount of light passing through the riflescope, everything will be the same as a 30mm scope because the size of the inner tube and lenses are the same. Could you perhaps describe the target image a little more for us. Where you in fact shooting towards snow covered areas, or water?

Also, do you keep both eyes open when you shoot through a riflescope. Do you wear a hat or a cap with a bill?

Do you have a sunshade on your riflescope?

Once we have answers to these questions, we can better understand your situation.

To answer TT's questions, I have -no- idea how he got the idea we had snow around. I never once mentioned the word. We actually had a smattering of it once in about 20yrs but it was melted the next day. "Cold" is a relative term- an Inuit would be stripped down to a teeshirt but 8-10 C here is cold enough for us North Island Kiwis.

Other variables: shooting at diamond aiming marks printed on std paper. I always shoot with both eyes open. The only interesting aspect was I was shooting from inside a little hut through a sound- suppression tunnel/chrono system. The backdrop to the electronic target frame is simply a hill.

I don't know if "my situation needs understanding" (to paraphrase TT). As I said, the thread title just caught my attention.

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